The present invention relates to the field of geophysical exploration and more specifically to the detection of sub-surface fractures with surface seismic data. The detection of fractures is of paramount importance in so-called "tight" petroleum reservoirs in which the primary determinant of well producibility is the presence of a connected network of fractures to convey fluid into the borehole. As is well known in the prior art, seismic shear waves are among the most sensitive tools available for detecting fractures in hydrocarbon reservoirs.
Previous shear-wave seismic acquisition and processing techniques have often relied on measurements of shear-wave splitting ("birefringence") to detect sub-surface fractures. To elaborate, aligned fractures induce horizontally-transverse anisotropy in the subsurface such that a vertically-incident shear wave splits into a fast mode polarized along the fracture direction and a slow mode polarized perpendicularly to the fractures. Shear waves are seismic vibrations that are polarized perpendicularly (or nearly perpendicularly in the case of an isotropic material) to their propagation direction, i.e., waves for which the vibrations occur perpendicularly to the direction of the wave's propagation. By measuring the shear-wave splitting (which is proportional to the velocity anisotropy), the location and density (the number of fractures per unit volume) of subsurface fracturing may be determined, because higher fracture densities cause a greater amount of shear-wave splitting. The use of four-component seismic acquisition (two orthogonal sources and two orthogonal receivers active during acquisition by both sources) is described in U.S. Pat. No. 4,803,666 to Alford, which is hereby incorporated by reference herein. Alford's technique entails the acquisition of a four-component data matrix to determine the symmetry planes of the medium. Alford describes the use of rotation algorithms to transform the seismic data into a symmetry-plane coordinate system which provides useful information about the orientation of the symmetry planes of the medium while also improving data quality. Knowledge of the orientation of the medium symmetry planes provided, e.g., by Alford rotation, is useful because the direction of open fracturing and that of the symmetry planes usually coincide. Measuring the symmetry plane orientation is therefore usually tantamount to measuring the fracture orientation, a parameter frequently difficult to determine from geological data and useful in reservoir management decisions.
A second consequence of horizontally transverse anisotropy caused by aligned vertical fracturing is a variation in seismic reflection amplitude at boundaries (such as a reservoir) as a function of profile azimuth caused by changes in the intensity of fracturing at the reflecting interface. This physical phenomenon enables fracture intensity to be ascertained with a relatively high vertical resolution by comparing the reflection amplitudes of the fast and slow shear-wave seismic sections. Both of these methods for fracture detection using shear-wave seismic data are described in detail in U.S. Pat. No. 4,817,061 to Alford et al., which is hereby incorporated by reference herein. Alford et al. describes the use of at least one source polarization along each source-receiver azimuth and receivers having matched polarizations.
However, these techniques may produce unsatisfactory results if the fracturing in the reservoir is relatively weak or if more than a single direction of open fracturing is present. Consequently, a method of seismic exploration which is able to characterize fracture intensity in the subsurface in even subtly fractured reservoirs would be advantageous.
Andreas Ruger, in his Ph.D. thesis, "Reflection Coefficients and Azimuthal AVO Analysis in Anisotropic Media", extended the theoretical treatment of split shear waves in the symmetry planes of a horizontally transversely isotropic ("HTI") medium or an orthorhombically anisotropic medium to the case of non-vertical incidence, deriving equations for the plane-wave reflection coefficients as a function of the waves' incident phase angle. This thesis is hereby incorporated by reference herein. AVO stands for amplitude variation with offset. At non-vertical incidence, i.e., by using offset seismic sources and receivers, it is possible to measure two additional amplitude attributes (reflection amplitude refers to the strength of the reflected signal observed at the receivers) in a seismic waveform, its reflection amplitude intercept and slope. The intercept is the projected zero-angle amplitude of the signal and provides information about the change in acoustic impedance and fracture density across the reflecting interface for shear waves. The reflection slope is the slope of a line fitted through the observed amplitudes as a function of the incidence angle of the waves and gives important information about the change in shear wave velocity and fracture intensity at the reflecting interface. This analytical insight into shear-wave reflection at non-normal incidence offers the possibility of obtaining more information about the elastic parameters (which are related to the fracture density) of the subsurface than previously possible if the numerous practical problems associated with the acquisition, processing, and interpretation of non-vertically-incident shear waves can be overcome.